Widely-used, portable small electric and electronic devices employ lithium ion rechargeable batteries, which have larger electromotive force and higher energy density compared to alkali rechargeable batteries, such as nickel-cadmium or nickel-hydrogen rechargeable batteries. With the recent improvement in performance and multifunctionality of these devices, further increase in battery capacity is demanded, and development of rechargeable batteries is being made actively.
Various researches have hitherto been made concerning anode active materials for lithium ion rechargeable batteries. Among such materials, lithium metal has been attracting attention as a material for an anode active material, for its capability of providing ample battery capacity. However, lithium metal has battery problems of precipitation of a large amount of dendritic lithium on the lithium surface upon charging to lower charge/discharge efficiency, which leads to short-circuiting the anode and the cathode, and also handling problems, such as instability and high reactivity of lithium per se. These problems impede practical use of lithium metal.
As a material for an anode active material in place of lithium metal, a carbonaceous material has been put into practical use. A carbonaceous material has a lower ratio of expansion/shrinkage due to charge/discharge compared to lithium metal or lithium alloys, but has a smaller battery capacity (theoretically about 372 mAh/g) compared to lithium metal.
In this regard, expected as large-capacity materials are silicon and tin. These materials have larger battery capacities compared to carbonaceous materials, and have been the subjects of active researches. However, these materials have a higher ratio of expansion/shrinkage due to charge/discharge and, when used as an anode active material, will disengage from the collector, leading to shorter battery life and larger irreversible capacity. In an attempt to solve such problem, silicon or tin is alloyed with other elements, or compounded with carbon, for suppressing expansion/shrinkage and reducing batter life shortening and irreversible capacity.
For example, Patent Publication 1 proposes an anode active material containing Li, Si, and C. This anode active material has good cycle characteristics, but a capacity as low as less than a half the capacity of a carbonaceous material.
Patent Publication 2 proposes an anode active material represented by M100-xSix (M=Ni, Fe, Co, Mn). This material is a silicide formed of a particular non-combustible transition metal element and a silicon metal, and thus has improved safety. However, the capacity of this material is as low as 672 mAh/g, which is about one sixth of the theoretical capacity 4200 mAh/g of elemental silicon.
Patent Publication 1: JPH07302588A
Patent Publication 2: JPH10294112A